/**
 * Created with IntelliJ IDEA.
 * Description:
 * User: yang
 * Date: 2022-04-28
 * Time: 18:55
 */
public class Main {
    public static void main(String[] args) {
        int[] arr={1,1,2,2,3,3,4};
        int ret=arr[0];
        for (int i = 1; i < arr.length; i++) {
            ret ^= arr[i];
        }
        System.out.println(ret);
    }
}
